1. Field of the Invention
The present invention generally relates to a ladder-type signal attenuator. More specifically, the present invention relates to a ladder-type signal attenuator employing an R-2R-type ladder resistor network and adapted for attenuating an analog signal under control by digital data to be a predetermined ratio of output/input.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing one example of a signal attenuator employing a conventional R-2R-type ladder resistor network which constitutes the background of the invention. An input terminal 1 is supplied with an analog input signal Sin and another input terminal 2 is supplied with a bias voltage Vb. The ladder network 5 comprises a plurality of stages, and in the embodiment shown, n stages, each having a combination of an input resistor 2R and an output resistor R. A switch circuit 4 comprises switches S.sub.l to S.sub.n corresponding to signal input terminals of the respective stages of the ladder network 5 and is adapted to be controlled in response to digital control data obtained from a control data generator 3. More specifically, the respective switches S.sub.l to S.sub.n are provided to selectively supply an analog input signal Sin or a bias voltage Vb to the corresponding signal input terminals of the ladder network 5 in response to the state of the bits b.sub.l to b.sub.n of the control data. An output signal Scout associated with the ratio of the output to the input determinable by the digital control data is obtained at an output terminal 6 from the ladder network 5.
As well known, in a signal attenuator employing an R-2R-type ladder resistor network, the ratio of the output (Scout-Vb) to the input (Sin-Vb) is expressed by the following equation (1). ##EQU1## where b.sub.1, b.sub.2, . . . b.sub.n are the control data obtained from the control data generator 3 and supplied to the switches S.sub.1, S.sub.2, . . . S.sub.n, respectively, shown in FIG. 1, and each assumes "0" or "1". More specifically, (b.sub.1 +2b.sub.2 + . . . +2.sup.n-1 .multidot.b.sub.n) in the equation (1) represents the control data, i.e. the decimal values with respect to the binary codes (b.sub.1, b.sub.2, . . . , b.sub.n) and the same are in succession incremented as 0, 1, 2, . . . , 2.sup.n-1 by changing in succession the binary codes (b.sub.1, b.sub.2, . . . , b.sub.n). Accordingly, (Scout-Vb)/(Sin-Vb) in the equation (1) becomes a straight line which changes at the pitches of 1/2.sup.n, as shown in FIG. 2.
On the other hand, such a variable resistor or a signal attenuator as shown in FIG. 3A, for example, is employed for volume adjustment, tone adjustment or balance adjustment in audio equipment and such a signal attenuator has a relation between the revolution angle of an adjusting knob and the output that can assume any one of curves A to E shown in FIG. 3B. Thus, volume adjustment of audio equipment requires a signal attenuator having such a characteristic of A curve or D curve as shown in FIG. 3B. However, a conventional signal attenuator employing a R-2R-type ladder resistor network could not achieve a characteristic of such A curve or D curve.
Furthermore, such a conventional signal attenuator as shown in FIG. 3A was suited for volume adjustment in audio equipment, for example, inasmuch as generally the larger the attenuation amount the smaller the output impedance thereof. However, such a signal attenuator employing an R-2R-type network as shown in FIG. 1 exhibits an output impedance which is approximately constant not withstanding a change in the attenuation amount. This means that the noise level at the output of the signal attenuator due to a thermal noise, for example, is constant and thus the noise level increases relatively to the signal as the attenuation amount of the signal increases, resulting in reduction of the signal to noise ratio. Such is extremely disadvantageous as compared with a conventional variable resistor-type attenuator in the case where such signal attenuator is utilized for volume adjustment in particular.